Hello,
Only once been on the Nice to Calvi ferry, that was at the end of May, we had booked on-line from Scotland it all seemed very busy!
Have been to Corse in June and September using the ferry Nice to Bastia again booked from Scotland....it always seemed very busy whatever time we travelled. Even the return in the middle of September was just the same scramble for a chair on the sun-deck! It is a popular place to visit fortunately not everyone travels on the GR routes. :-)